1. Field of the Invention
This invention is directed to the application of nanoweb polyimide separators in electrochemical cells which may be lithium (Li) and lithium-ion (Li-ion) batteries.
2. Description of the Related Art
An important practical aspect of modern energy storage devices is ever-increasing energy density and power density. Safety has been found to be a major concern. Commercially available Li-ion batteries typically employ microporous membranes based on polyethylene and/or polypropylene as a battery separator. These membranes begin to shrink at 120° C., limiting the battery fabrication process, the operating temperature of the battery, and the power available from the battery.
The requirements for choosing an improved separator for Li-ion batteries and other high energy density electrochemical devices are complex. A suitable separator combines good electrochemical properties, such as high electrochemical stability, low charge/discharge/recharge hysteresis, good shelf life (low self discharge), low first cycle irreversible capacity loss, with good mechanical aspects such as strength, toughness and thermal stability.
Investigations concerning known high performance polymers for use as battery separators have been undertaken. One such class of polymers has been polyimides.
The Handbook of Batteries, David Linden and Thomas Reddy, ed., McGraw-Hill, (3rd edition), 2002, describes first cycle discharge capacity loss as an important criterion in secondary batteries (P. 35.19). Also stated is that non-woven separators have been found in general to exhibit inadequate strength for use in Li and Li-ion batteries. (P. 35.29). For this reason, low-melting polyolefin based microporous films tend to be used as separators in Li and Li-ion batteries. However, polyolefin based microporous films are not thermally suited to the high temperatures occasionally associated with rapid discharge end uses, or end uses in high temperature environments.
Huang et al., Adv. Mat. DOI: 10.1002/adma.200501806, disclose preparation of a mat of polyimide nanofibers by electrospinning a polyamic acid that is then imidized to a polymer represented by the structure.
The mat so prepared is then heated to 430° C. and held for 30 minutes, thereby producing an increase in strength. No mention is made of battery separators.
Honda et al., JP2004-308031A, discloses preparation of polyimide nanowebs by electrospinning polyamic acid solution followed by imidization. Utility as a battery separator is disclosed.
Nishibori et al., JP2005-19026A, discloses the use of a polyimide nanoweb having sulfone functionality in the polymer chain as a separator for a lithium metal battery. The polyimide is described as soluble in organic solvents and the nanoweb is prepared by electrospinning polyimide solutions. No actual battery is exemplified. Heating of the nanoweb to about 200° C. is disclosed.
Jo et al., WO2008/018656 discloses the use of a polyimide nanoweb as battery separator in Li and Li-ion batteries.
EP 2,037,029 discloses the use of a polyimide nanoweb as battery separator in Li and Li-ion batteries.
A need nevertheless remains for Li and Li-ion batteries prepared from materials that combine good electrochemical properties, such as high electrochemical stability, low charge/discharge/recharge hysteresis, good shelf life (low self discharge), low first cycle irreversible capacity loss, with good mechanical aspects such as strength, toughness and thermal stability.